But the report on the initial findings of the NTSB investigation offered no pinpointed cause of the fire that contributed to the global grounding of 787s almost two months ago.

"The NTSB's investigation into the probable cause of the 787 battery fire at BOS is continuing. The NTSB is also continuing to review the design, certification, and manufacturing processes for the 787 lithium-ion battery system," the independent government agency said in the report.

In its last briefing a month ago, the NTSB identified a short circuit on a single cell in the eight-cell JAL battery that had sparked overheating.

Another burned battery incident, on an All Nippon Airways 787 that was forced into an emergency landing in Japan, led regulators to ground all 50 787s in use in mid-January.

In a fresh sign of the complexity of the international investigation of the batteries, NTSB chief Deborah Hersman said that the agency would hold a forum and a hearing in April.

The forum in mid-April will focus on lithium-ion battery technology and transportation safety.

An investigative hearing later in April will cover the battery system's design and certification by the Federal Aviation Administration (FAA), a certification process that Hersman has criticized.

"The information developed through the upcoming forum and the hearing will help the NTSB and the entire transportation community better understand the risks and benefits associated with lithium batteries, and illuminate how manufacturers and regulators evaluate the safety of new technology," Hersman said in a statement.

Both hearings will be webcast live and open to the public.

The NTSB investigates civil aviation accidents but the FAA is responsible for deciding whether the 787 can return to service.

The FAA is studying a Boeing plan to address the battery problem submitted on February 22.

According to aviation research firm Leeham News and Comment, the FAA was expected to give Boeing the green light to begin implementing its proposed fixes Friday or next week.

US Transportation Secretary Ray LaHood told The Wall Street Journal Wednesday that he needs more evidence to be convinced that Boeing's proposed fixes are adequate to ensure the planes are safe to fly.

"I have made it very clear that I want a thorough review" of the Boeing plan, LaHood said.

"I am going to ask a lot of questions" before a final decision is made, said the transportation secretary, who oversees the FAA.